Implanted devices provide therapy for many kinds of cardiac problems. Their effectiveness results from precise sensing of the electrical waveforms generated by the heart during ongoing cardiac cycles and precise application of remedial electrical pulses in a manner that conserves an onboard battery for many years.
Implanted devices have conventionally relied on relatively invasive installation procedures to attain the precision needed for effective cardiac control while simultaneously sparing the implanted battery. That is, in order to be practical, conventional electrodes are meticulously positioned—often across heart valves—to be in contact with cardiac tissue in the most inviolate recesses of the heart. Fortunately, this cardiac invasiveness has proven to be relatively safe. Implanted cardiac leads are now taken for granted. Still, problems can arise after electrode placement, and lead placement inside the heart is avoided if there is a workable alternative.
Another problem with conventional lead placement for implanted cardiac devices is the level of skill and sophistication of equipment needed to achieve good placement. An implantation procedure typically lasts a couple of hours and requires a small team of skilled practitioners threading leads transvenously under the assistance of x-ray fluoroscopy. In other words, conventional lead placement is relatively expensive. Nonetheless, this expensive procedure is currently the standard and the resulting conventional implanted cardiac devices successfully treat many cardiac ailments.
Perhaps the gravest of these cardiac ailments is ventricular fibrillation. The condition is so serious that it sometimes goes by other names, such as “cardiac death” and even “sudden cardiac death.” In ventricular fibrillation, the heart rate in the ventricles becomes ineffectively rapid because the electrical activity controlling the ventricles has become completely chaotic. The heart beats so quickly and chaotically that the ventricles effectively tremble instead of pumping blood. An implanted cardiac device can be programmed to respond to ventricular fibrillation by applying a strong electrical shock that stops all erratic electrical activity allowing a normal cardiac rhythm to ensue.
A similar malady, ventricular tachycardia, occurs when the electrical impulses controlling the ventricles remain orderly but occur far too rapidly to effectively pump blood. Ventricular tachycardia can quickly turn into ventricular fibrillation. Conventional implanted cardiac devices can treat ventricular tachycardia. If ventricular tachycardia is sensed, an implanted device can apply rapidly paced beats—anti-tachycardia pacing—at a pace that is even more rapid than the tachycardia, thereby overcoming the heart's own abnormal rate. When the artificial pacing is stopped, the heart often returns to a normal rate and rhythm. Sometimes anti-tachycardia pacing does not work, so a second tier remedy is applied in the form of cardioversion shocks that are timed to coincide with features of the heart's inherent rhythm in order to stop the ventricular tachycardia and bring the rate and rhythm back within normal parameters.
To achieve the advantages of a conventional implantable cardioverter-defibrillator (ICD) as just described but avoid invasive and expensive installation of the ICD, various conventional systems have been attempted that utilize subcutaneous components. A subcutaneous ICD system aims to have electrodes that do not physically invade the heart to theoretically provide the best of both worlds: defibrillation capability with relatively quick and easy installation of the device. But in practice, such conventional subcutaneous or semi-subcutaneous systems have suffered several drawbacks. First, since the electrode for originating a defibrillation current is not in physical contact with the heart, more current is needed to perform defibrillation, i.e., these systems result in a higher “defibrillation threshold”—which is harder on the patient and on the battery when defibrillation is applied.
Second, conventional subcutaneous installations result in a positioning of electrodes that is at variance with optimal electrode locations for sensing and stimulating the heart—e.g., the shocking pathway(s) achieved impinge the heart in non-optimal planes. This increases defibrillation thresholds.
Third, to keep the system subcutaneous and noninvasive, the sensing electrodes, like the shocking electrodes, are also not in physical contact with the heart. Thus, sensing is more difficult and prone to noise interference.
There is a need for subcutaneous and semi-subcutaneous implantable systems that enable defibrillation thresholds that are comparable to those of conventional ICDs while also providing improved sensing.